This invention relates to electric heating elements, for example for use in liquid heating vessels, such as kettles, rice cookers, coffee makers, etc. The invention relates particularly to heating elements which comprise an electrically heated conducting track provided over a substantially planar metal substrate.
This type of heating element is increasingly being used in electric kettles, where it provides the advantage that cleaning the inside of the kettle is easier, and it may be possible to boil a small quantity of water, since a smaller quantity of water is required to cover the heating element than is required for conventional immersion elements. Safety requirements dictate that electric kettles require two protection devices to ensure that the electrical supply to the heating element is broken in the event of overheating of the electric kettle (for example if the steam sensitive cut-off switch for boiling fails, or if the kettle is turned on with no water in it). Conventionally, the two overheat protection devices have been integrated into a control unit of the electric kettle, and one or both of these overheat protection devices may comprise a bimetallic switch which switches off when a bimetallic strip reaches a predetermined temperature. Additionally, or alternatively, portions of the control housing may be formed from a plastic which melts at a predetermined temperature so that in the event of failure of all other overheat protection devices, the body of the control housing melts resulting in movement of components causing disconnection of the electrical supply to the heating element. If this melt-down protection is employed, only one thermal protection device in the form of a bimetallic switch may be required.
EP 0 715 483 which corresponds substantially to U.S. Pat. No. 5,793,929, discloses an electric heating element comprising a conductive heating track provided over a substrate, and extending between two contact terminals. The track comprises a first portion which extends around the circumference of the heating element and is an unheated portion of the track. This first portion is connected in series to a second, inner, heating portion of the track. The unheated portion of the track acts as a thermal fuse which breaks the connection between the two contact pads in the event of overheating of the heating element.